This invention relates to high strength, high temperature corrosion resistant structures and their processes of preparation. More particularly, this invention relates to structures which are capable of sustaining heavy loads and withstanding highly corrosive environments at greatly elevated temperatures and pressures for hours or even days.
Previously considerable difficulty had been experienced in providing structures which retain their structural integrity at temperatures in excess 2,000 degree centigrade while sustaining substantial structural loads. In general, materials which can withstand environments under which most materials catastrophically oxidize do not have the structural strength to withstand substantial loads. Conversely, materials which have substantial structural strength at elevated temperatures are generally subject to catastrophic oxidation. These material limitations have substantially hampered developments and performance in a number of fields including, for example, that of rocketry. Rocket thrust chamber assemblies have, for example, been operated at less than optimum conditions so as to keep the operating temperatures within the limits of the material which had heretofore been available for use in the construction of thrust chambers and nozzles. Even when less than optimum operating conditions were used, the life of previous thrust chamber assemblies was generally limited to a very few minutes before catastrophic structural failure occurred.
These and other difficulties of the prior art have been overcome according to the present invention which provides a high temperature, high strength corrosion resistant structure which is capable of withstanding hot highly corrosive environments for several hours while supporting substantial structural loads.
According to the present invention, chemical vapor deposition procedures are utilized to form successive layers of a structure under conditions such that the layers are bonded together by an interlayer which is an admixture of the two adjacent layers. A first corrosion resistance layer is provided which is capable of withstanding highly corrosive environments at temperatures in excess of 1,500 degrees centigrade and preferably an excess of 2,000 or 2,200 degrees centigrade. The corrosion resistant layer is bonded through an interlayer to a high strength layer. The high strength layer exhibits tensile strengths in excess of 5,000 and preferably 15,000 pounds per square inch at temperatures in excess of 1,500 and preferably in excess of 2,000 degrees centigrade. In general, the structures according to the present invention are composed of platinum group metals such as, for example, iridium, and refractory materials, preferably metals such as, for example, rhenium. The structures may, if desired, be coated with high temperature ceramic materials such as, for example, hafnium dioxide or zirconium dioxide.
When thrust rockets are operated at the optimum oxidizer/fuel ratio, the temperature of the exhaust gas may be as high as 2,000 to 2,500 degrees centigrade. This is the condition at which the highest efficiency is achieved. It has heretofore been impossible to operate at these conditions because of the limitations imposed by the materials of construction for the thrust chamber assembly. An additional factor is of substantial significance in this area. Thrusters must be cycled on and off many times over their useful lives. The thrusters may cool between cycles to very low temperatures. Further, thrusters must withstand the forces imposed on space vehicles during launch and in use. The materials from which thrusters are constructed must withstand both structural and thermal shock. Brittle materials tend to fail rapidly because of an inability to withstand this shock. Premature failure because of cracking under hock greatly limits the useful life and reliability of the thruster. Very few materials or material combinations are capable of withstanding the shock loads, the structural loads, and the catastrophically corrosive conditions which are imposed on a rocket thrust chamber assembly. Previously the efficiency of thrust chambers had been substantially compromised by limiting the operating temperature to less than approximately 1300 degrees centigrade and by limiting the number of cycles to which the thruster could be subjected.
Thrust chamber assemblies are used in space for the maneuvering of satellites and otherwise. Improving the efficiency of the thruster and extending its life provides a substantial number of new alternatives in space applications. The same thrust can be obtained with less fuel and thus less weight. The savings in weight can be distributed between additional fuel and additional payload. Extending the life of the thruster and the number of cycles which it can withstand prolongs the useful life of the satellite or vehicle upon which it is mounted. Increasing the effective fuel capacity of the vehicle also extends its useful life.
Many applications exist outside of the space field for structures which are capable of withstanding shock, and high structure loads in highly corrosive high temperature environments. The absence of such structures limits or precludes the use of some reactions in the chemical process industry Such structures find application in propulsion systems and prime movers other than rockets. Other fields such as nuclear, metallurgical, and the like also require such structures to optimize or make possible various operations.
The platinum group metals and particularly iridium are capable of withstanding highly corrosive high temperature conditions, these platinum group metals do not, however, possess sufficient strength at high temperatures to withstand substantial loads. Certain refractory materials retain their strength at temperatures as high as 2,500 degrees centigrade or higher. Such refractory materials, however, are generally subject to rapid catastrophic oxidation at elevated temperatures.
Iridium is recognized as being the most corrosion resistant element known. It was, however, previously believed to be very brittle so that any significant elongation or flexing would cause it to fracture. It has been discovered that it is possible to form ductile iridium in useful shapes through the use of chemical vapor deposition techniques. These shaped objects are highly corrosion resistant and are adapted to be used in an oxidative environment at temperatures in excess of approximately 1500 degrees centrigrade. Iridium alloys, formed through the sequential or codeposition of iridium and other materials also exhibit ductile characteristics.
A structure which is capable of withstanding high temperature, high stress, shock and catastrophically corrosive conditions can be produced according to the present invention by providing a corrosion bearer layer comprised of platinum group metal which is bonded through an interlayer to a layer of refractory material. The interlayer is an admixture of the corrosion bearer layer and the load bearing refractory layer. The corrosion resistant layer and interlayer protect the load bearing layer from catastrophic corrosion. Particularly, advantageous results have been achieved through the use of elemental iridium as the corrosion resistant layer and rhenium as the load bearing layer. Iridium is conveniently deposited on a mandrel which is subsequently removed. The deposit is thick enough to be separable from the mandrel. Rhenium is next deposited over the iridium layer under conditions which cause the formation of a metallurgical bond between the iridium and rhenium. The metallurgical bond takes the form of an interlayer in which the iridium and rhenium are admixed. The properties of the interlayer transition from those of the iridium to those of the rhenium, with the melting point of the interlayer increasing as it progresses from the iridium to the rhenium. The corrosion resistances of the layer decreases as it approaches the rhenium but remains closer to that of the iridium throughout most of the interlayer. The melting point of the transition layer increases faster than the corrosion resistance decreases as its composition moves from pure iridium toward pure rhenium. When the composition of the interlayer reaches approximately 80 percent rhenium, the corrosion resistance becomes insufficient to protect the structure from catastrophic corrosion. The mandrel is removed leaving a free standing structure. The interlayer is ductile. The structure is very shock resistant and not subject to cracking The metallurgical bond is very tight so that there is no tendency for the layers to delaminate. It has been found that the reverse procedure of depositing iridium on rhenium does not produce a metallurgical bond which is as satisfactory.
The life of the structure can be further prolonged by the application of a ceramic coating on at least the surface of the iridium which is exposed to the hot corrosive environment. The ceramic coating is brittle and will crack but usually not spall off, particularly under repeated cycles, but will provide some substantial protection for at least the initial part of the usage of the structure. Suitable ceramic coatings include, for example, hafnium dioxide and zirconium dioxide. The ceramic coating is formed by chemical vapor deposition procedures and is joined to the corrosion resistant layer under conditions which cause the formation of a bonding interlayer between the ceramic and the corrosion resistant layers. Typically the ceramic layer is deposited first on a mandrel under conditions where the ceramic layer does not bond to the mandrel and the corrosion resistant layer is next deposited over the mandrel onto the ceramic layer.
Chemical vapor deposition procedures are known and have been used for the better part of a century for forming various coatings. In general it is a method of plating on an atom by atom basis in which a gaseous compound of the material to be deposited is flowed over a heated substrate, resulting in the thermal decomposition or reduction of the compound and the subsequent deposition of the material onto the surface of the heated substrate. The parameters which must be controlled for successful reliable operation include the choice of gaseous compound, the concentration of the compound in the gas, the gas flow rate, the gas pressure, the nature of the substrate material, the geometry of the substrate, the temperature of the substrate, and the geometry of the reaction chamber. The nature of the deposit may be controlled by controlling these parameters. The crystal form may, for example, sometimes be changed by changing the gaseous compound. The degree of adherence to the substrate may be controlled, for example, by adjusting the temperature of the substrate. The deposits formed by chemical vapor deposition are generally very pure. Materials may be codeposited depending upon the composition of the gas which is supplied to the reactor. If the deposit is thick enough it may be separated from the mandrel to leave a free stranding structure. Both metallic and non-metallic materials may be deposited using these techniques and they may even be codeposited.
For the sake of convenience in describing and defining the process and the structure the structure has been referred to as comprising various layers of materials. If desired, however, it is possible with chemical vapor deposition techniques to provide a continuous variation in the composition of the structure from one pure material at one outer surface to another pure material at the opposed outer surface. Also, the variation in the composition need not be continuous. If ceramics are used on one outer surface it is possible to provide such a continuous variation in composition across three or more materials, not all of which are metallic.
Such graded deposits have no discontinuity and thus no stress concentration due to a mismatch of thermal expansion rates. Graded deposits are produced by varying the composition of the reactive gas as the deposit builds up. The description and definition of the process and structure is intended to include such graded deposits.